1. Field of the Invention
The invention belongs to the automotive arts. Specifically, the invention concerns a brake system for a motor vehicle and a method for transmitting data in an electrically controlled brake system for a motor vehicle.
The brake systems of modern motor vehicles are subject to a variety of new requirements--such as anti-locking systems ABS, drive stability systems, traction control systems, so-called intelligent cruise controllers, brake assistants, etc.--and they are subject to a demand for reduced assembly and maintenance costs, which are very considerable for present-day brake systems. These new requirements and demands, have led to the development of purely electric brake systems, which are also known as "brake-by-wire" systems. (See, for example, German published patent application DE 195 11 287 A1; and journal mot 20 (1995), p. 46).
With such electric brake systems the driver is separated from the brake in respect of the force exerted, i.e. the braking moment demanded by the driver is no longer transmitted directly as force through a hydraulic system but only as a signal over an electrical connection. The signal is used to control an electric brake actuator which by means of a source of electric power generates force at a brake which in turn produces the required braking moment through a friction element. In the case of a disk brake the force generating the braking moment through friction is designated as the brake application force.
A completely electric brake system is described, for example, in the commonly assigned, copending application No. 08/843,887, filed Apr. 17, 1997. There, the brake actuator is attached directly at the wheel of the motor vehicle in order to avoid transmitting a brake application force over long distances in the motor vehicle, while the brake controller and the detection of the braking moment demanded by the driver are arranged at a central position, for example in the vicinity of the brake pedal. Thus the brake system is distributed over the entire motor vehicle and, since the brake system forms a central safety function of the motor vehicle, stricter safety requirements must be met in respect of the electrical transmission of the braking moment demand. The transmission must be fail-safe and fault-tolerant, i.e. errors occurring during the communication must be detected reliably by the brake system and suitable strategies must be available for handling errors. In addition, all subscribers in the electrical communication process must be able to recognize faulty behavior on the part of another subscriber. In particular, a minimum braking capability must be guaranteed even if communication is interrupted.